1. Field of the Invention
The present invention presents a method for reducing eddy currents in a magnetic resonance system, especially a method for reducing eddy currents caused by the gradient magnetic field used in an open magnetic resonance system.
2. Description of the Prior Art
A common open magnetic resonance system (Open MRI) typically has a yoke, pole plates, a magnet (permanent magnet or superconducting magnet), an anti-eddy current device and a Rose ring. Referring to FIG. 1 and using a common open MRI as an example, magnet 20 is set between pole plates 10 and 30 to generate a main magnetic field. Rose ring 40 is set under the pole plate 30 for shimming. The gradient coil 60 is provided in the magnetic resonance system to generate a gradient magnetic field. Anti-eddy current device 50 is set between the pole plate 30 and the gradient coil 60 to reduce the eddy current caused by the gradient magnetic field.
With the continuous development of MRI pulse sequence, the requirements on gradient amplitude and slew rate of the gradient magnetic field are increasingly demanding, which results in stronger eddy currents. These eddy currents not only increase the ramping time of the gradient magnetic field, but also tremendously affect the homogeneity of the main magnetic field and ultimately result in decreased quality of the magnetic resonance images.
U.S. Pat. No. 5,283,544, Chinese Patent Applications No. 01245762.0 and No. 94222373.X separately propose an anti-eddy current device for use in a magnetic resonance system to reduce eddy currents wherein the eddy currents are reduced by optimizing the structure of the anti-eddy current device to shorten the path of the eddy currents. Chinese Patent Application No. 99118614.1 proposes another anti-eddy current device made of a non-crystal material, which has a higher resistance and is much thinner compared with the silicon steel laminations adopted in the conventional anti-eddy current devices, and can effectively reduce the eddy current.
These two anti-eddy current devices can be used in combination to achieve a better reduction of the eddy current.
However, these anti-eddy current devices are only improved in terms of their own structures or materials without taking into account how the magnetic field of the anti-eddy current devices is distributed in a specific working environment, which does not permit these anti-current devices to achieve the expected effects or to be made full use of in actual operation.